Abstract: The biodegradabilities of biodiesels prepared from cotton seed oil via methyl or ethyl were investigated by the OECD 301B Modified Sturm Method. In aquatic aeration cultures, pure cotton seed oil methyl and ethyl esters biodegradability rates was 99.7% and 99.1%, respectively, after 28 days; Biodegradability rates of cotton seed oil methyl and ethyl esters in samples of B50 was 93.8% and 95.7%, respectively, after 28 days. Biodegradability rates of cotton seed oil methyl and ethyl esters in samples of B20 was 80.8% and 81.9%, respectively, after 28 days, but only 49.9% of fossil diesel was decomposed at the same condition. The more the cotton seed oil methyl and ethyl esters in mixtures were, the easier the biodegradability was. The biodegradability of the fuel mixture was enhanced by adding biodiesel. The research contributes to understand the decomposition characteristics of biodiesel via cotton seed oil, and also helps to control environment pollution.